Why the US and much of Europe are shivering in the cold

The Arctic Oscillator has dropped a blanket of cold air over North America and …

Those of us shivering through extended stretches of subfreezing temperatures might be forgiven for getting a bit impatient for the onset of more significant global warming. And, if you're reading Ars, chances are good that this describes you, as the US and Europe have been blanketed in an unusual chill. Ironically, as these inhabited parts shiver, the atmospheric system that's causing it, the Arctic Oscillation, has covered Greenland and the Arctic Ocean with air that's equally as extreme, but in the warm direction.

The folks who run the National Center for Atmospheric Research have a great rundown of the details of the AO Oscillation. In short, high pressure in the Arctic forces the jet stream south, and it drags cold air with it, chilling North American and northern Eurasia. In its opposite mode, those same regions tend to be much warmer. Right now, we're in such an extreme high-pressure event that the readings have run off the scale of NOAA's AO index. Fortunately for those hoping to warm up a bit, the AO is a weather event—it often changes states multiple times within a single season, and there's no clear evidence linking its behavior to climate trends.

The NCAR site also points out one of the reasons why people are making a big deal out of this one: we tend to think short-term when it comes to our surroundings. We haven't had an AO event this severe since 2003, and the high pressure mode has been relatively rare since 1990, so many places have simply gotten used to not having an Arctic blast during the winter. The fact that November was unusually warm in the US, Canada, and Europe probably doesn't help matters, either.

When it comes to longer-term impacts, this strong phase of the AO may significantly alter the dynamics of the Arctic Ocean's ice pack, which responds both to weather events and climatic trends. Most of the Arctic Ocean freezes up during the winter, but the warm air present may limit the extent and thickness of solid ice sheets, meaning a lot of this year's freeze is likely to simply remelt next summer. At the same time, however, the wind patterns that are prevailing will drive less of the ice out of the Arctic Ocean, which may preserve some of the older, more robust multiyear ice.